Synthetic ATF. It is much less hygroscopic than mineral based oils so you will note much less corrosion, as you have some humidity there, too, just as I do.
Use it in the primary case, too. It's ideal for this use.
Never noted any difference in the way synth. oil attacks gaiters, in fact, I think it is less agressive than most mineral based oils. In fact, I spray the gaiters with silicone spray before fitting them, inside and out.
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